The present invention relates generally to improvements in signal processing systems, and more particularly to improved techniques and designs for eliminating interference or otherwise undesirable signal components from serial data samples to be processed by a moving target indicator type radar system.
As is well known in the art, desired information received by a communication, sonar or a radar system is frequently not isolated by itself, but is found in the presence of unwanted signals. These unwanted signals typically vary much more slowly with time than the desired signals. Thus, these unwanted signals generally are correlated from data pulse sample to data pulse sample. With specific respect to a moving target indication radar system, radar echos from non or slowly moving radar reflectors such as ground clutter, sea returns, and reflections from wind driven interference such as rain and chaff, all generate echos which have frequency components which are relatively slowly changing withg respect to a moving target. Thus, the echos from this radar clutter are correlated between successive radar pulse samples, i.e., these echo components appear the same in adjacent data pulse samples. Accordingly, since rapidly moving reflectors, i.e., targets, do not correlate from data sample to data sample, it is possible to significantly enhance the moving target signal by removing the correlated signal components resulting from sea, ground, rain, and chaff echos.
Prior art MTI systems utilize cancellers in order to decorrelate the output of a data pulse sample from an adjacent data sample. The typical 2-pulse adaptive canceller operates to phase shift and amplitude weight one of the data samples, and then to subtract this phase shifted and weighted data sample from another of the data samples. Such systems work well to eliminate correlated interference when only one narrowband interference source is present. However, when multiple and/or wideband interference sources are involved, a multiple degree-of-freedom system is needed to cancel the correlated components of the data samples. Theoretically, if N independent interference sources are present in a signal environment, the interference signals may be cancelled by multiple cancellers fed by inputs from separate data sample pulses. In practice, however, it has been found that effective cancellation cannot be obtained unless the data pulse sample inputs are independent, i.e., decorrelated from one another, in order to prevent the reintroduction of signals which have been cancelled in a previous canceller's circuit.
A typical prior art adaptive moving target indicator system for providing optimum clutter cancellation utilizing independent auxiliary data samples, requires N(N-1)/2 cancellers, where N is the number of pulses used in the MTI. A prior art MTI structure of this type is shown in FIG. 1. This configuration utilizes Gram-Schmidt processing using series iterative cancellation. FIG. 1 is broken up by means of dashed lines in order to show a 2-pulse canceller configuration, a 3-pulse canceller configuration, a 4-pulse canceller configuration, and a 5-pulse canceller configuration. The extension to higher order configurations is clear. A standard tapped delay line 10 is utilized in order to obtain multiple data samples. Each delay line provides a delay equal to the interpulse delay T between radar pulss. Accordingly, for a 2-pulse cancellation system, a radar pulse return signal is applied from the terminal 14 directly to the auxiliary input of canceller 16 and indirectly through an interpulse delay element 12 to the main input of the canceller 16. The result of this cancellation for a series of pulse samples numbered consecutively 1, 2, 3, 4, . . . is obtained on line 18 and, for the first two pulses numbered 1 and 2, is equal to 1.perp.2=A, i.e., the components of the pulse sample 1 that are perpendicular to the components of pulse sample 2, or more succinctly stated, pulse sample 1 decorrelated from pulse sample 2. The 2-pulse canceller is sufficient to cancel one source of narrowband interference.
In order to cancel additional or wideband sources of interferecne, 3-, 4-, and 3-pulse cancellers are utilized. In FIG. 1, a 3-pulse canceller is formed by adding a second interpulse delay segment 20 in conjunction with a second canceller 22 and a third canceller 24. In operation, pulses are applied from the terminal 26 in the delay line 10 directly to the main input of the canceller 22 and then indirectly through the interpulse delay element 20 to the auxiliary input of the canceller 22. The resulting output from the canceller 22 is pulse 3 decorrelated with pulse 2 and is represented as 3.perp.2=B. B is then applied on the line 30 to the auxiliary input to the canceller 24. The ouput A on line 18 is applied to the main input of the canceller 24. The output from the canceller 24 on line 32 is A decorrelated from B and is represented by A.perp.B. In order to obtain a 4-pulse canceller, a third interpulse delay element 34 is utilized in the delay line 10 and three additional cancellers 36, 38, and 40 are included. In operation, pulses are taken from the terminal 42 in the delay line and are applied directly to the main input of the canceller 36. The signal at point 14 in the delay line is applied via the line 44 to the auxiliary input to the canceller 36. In the example shown in the figure, pulse 2 in the pulse sequence is applied to the auxiliary input of the canceller 36 while pulse 4 is applied directly to the main input of the canceller 36. The result is pulse 4 decorrelated from pulse 2 and is represented by 4.perp.2=C on line 48. C is then applied to the main input for the canceller 38. B from the canceller 22 on line 30 is applied to the auxiliary input for the canceller 38. The resulting output from the canceller 38 is on line 50 and is C decorrelated from B and is represented by C.perp.B. C.perp.B is applied via the line 52 to the auxiliary input to the canceller 40. The signal on line 32, A.perp.B, is applied to the main input of the canceller 40. The resulting output from the canceller 40 is on line 54 and is (A.perp.B).perp.(C.perp.B).
Finally for a 5-pulse canceller, a fourth interpulse delay element 60 is utilized in the delay line in conjunction with the additional cancellers 62, 64, 66, 68. The fifth pulse is applied from the terminal 70 directly to the main input of the canceller 62. Pulse 2 from terminal 14 in the delay line is applied via the line 71 to the auxiliary input to the canceller 62. The resulting output from the canceller 62 is found on line 72 and is pulse 5 decorrelated from pulse 2 and is represented by 5.perp.2=D. The output from the canceller 62, i.e., D, is applied to the main input of the canceller 64. The output B on line 30 from the canceller 22 is applied to the auxiliary input of the canceller 64. The resulting output from the canceller 64 is found on line 74 and is D decorrelated from B and is represented D.perp.B. The output D.perp.B is applied to the main input of the canceller 66. The output C.perp.B on line 52 from the canceller 38 is applied to the auxiliary input for the canceller 66. The resulting output from the canceller 66 is found on line 76 and is (D.perp.B).perp.(C.perp.B). This output on the line 76 is applied to the auxiliary input to the canceller 68. The output (A.perp.B).perp.(C.perp.B) is applied on line 54 to main input to the canceller 68. The resulting output from the 5-pulse canceller is found on line 80 and is [(A.perp.B).perp.(C.perp.B)].perp.[(D.perp.B).perp.(C.perp.B)].
It can be seen from the above discussion of FIG. 1 that in order to obtain optimum clutter cancellation, a significant number of cancellers are required. In this regard, as the number of pulses in the pulse cancellation system increases, the number of required cancellers rapidly increases, i.e., N(N-1)/2 cancellers are required for N pulses. In the example shown in FIG. 1 for a 5-pulse MTI, 10 cancellers are required. This large number of cancellers required to obtain an optimum clutter cancellation is so costly that it is essentially impractical.